It is known that the operating machines used mainly in the agricultural sector or on building sites is equipped with two separate brake circuits that can activate the brakes of the rear wheels on the left and right-hand sides independently and which are each operated by separate pedals, both of which are located in the driving position.
This feature is used by drivers to take the operating machine around turns quickly and with a very limited radius of curvature: the driver basically presses the brake pedal that faces the direction of bending, by braking or blocking the rear wheel of the side of the machine facing this direction. The rear wheel of the other side continues to drive the machine, which therefore basically turns on itself around the braking wheel, moving progressively in the direction of bending.
Nevertheless, the two brake circuits must also brake the operating machine when it is moving along a road that is open to traffic for normal transport, slowing it down or blocking its movement as necessary.
In order to obtain this action, the driver has to simultaneously press both pedals controlling the respective brake circuits, so that the braking action takes place on the wheels of both sides of the operating machine equally.
In order to facilitate the driver's action, the pedals can also be joined together using removable devices which are only used when on the road and which are removed when the machine has to operate on sites or in fields.
There must not be any imbalance in the power of the braking action on the two sides of the operating machine during transportation as on roads with normal traffic the machine is capable of reaching notable speeds of around 40/50 km/hr, and an imbalance, that is a stronger braking action on one side than on the other, may cause the operating machine to swerve dangerously.
Imbalances in the braking power can be caused by a variety of elements that make up the braking system on an operating machine.
One of the elements that can cause an imbalance between one brake circuit and the other is the manufacturing tolerance of the individual components on each circuit, in particular the machining on the bodies of the pumps or the selector valves, the sliders and the ports through which the pressurised oil that operates the brakes flows.
Other elements that may cause an imbalance are the springs used in the pumps and the valves, springs which may have a slightly different modulus of elasticity from one another even if the springs on one brake circuit and on the other brake circuit are substantially the same as one another.
A further element that can cause braking imbalance is imperfect alignment of the pedals controlling each brake circuit on the operating machine, alignment which must be as precise as possible when braking on the road.
Another feature that is required of the braking systems of operating machines is that they be easy to operate, as similar as possible to those on a normal car.
Basically, the stroke of the pedals controlling the brake circuits must be considerably limited and the force that the driver has to apply to the pedals must be limited and easy to adjust.
At present, pumps which move considerable amounts of pressurised oil are used to limit the stroke of the brake pedals.
However, the drawback with using this type of pump is that a considerable physical effort is required of the driver in order to move this amount of oil and thus operate the brakes.
Servo-controls are therefore fitted onto the brake circuits in order to limit the force that has to be applied to the pedals. These magnify the force applied by the driver, but have the further drawback of being very expensive, thus increasing the costs of producing and purchasing operating machines.
As an alternative to fitting servo-controls, pumps are also used to limit the operating force on the pedals. These move limited amounts of oil, but this solution has the further drawback of increasing the stroke that the pedals have to complete in order to operate the brakes efficiently.
In order to compensate the pressure differences that can form between the two brake circuits, structural solutions are adopted which require a pump to be fitted on each of the two brake circuits respectively: the pumps are positioned in parallel to one another and reciprocal connection ducts are provided between them that allow the two pumps to communicate with one another or they can be isolated using opening and closing devices that are controlled by the pump pistons during their stroke.
According to a further well-known technical solution, an automatic pressure or displacement compensation device is arranged between the brake circuits which comprises a cylinder whose ending parts are connected to each brake circuit via respective ducts and inside which there is a chamber in which a central piston moves in contrast to elastic members; the central piston divides the chamber into two semi-chambers and, moved by the thrust of the higher pressure present in one circuit than the other, it slides in order to adjust the volumes of the semi-chambers and thus compensate the pressure between the two circuits.
In order to overcome the drawbacks found with the specific use of pumps, for some time now preference has been for the use of braking systems with selector valves which distribute pressurised oil supplied from a source, for example an accumulator.
In this case, the amount of oil moved in the brake circuits is unrelated to the dimensions of the selector valves used and these do not have to push the pressurised oil towards the brake circuits, but simply distribute it towards it: as a result, both the pedal strokes and the force that must be applied to the brake pedals are significantly lower than is required if pumps are used.
A well-known braking system that comprises two hydraulic circuits that can be operated independently from one another or jointly has a selector valve fitted on both the right-hand and on the left-hand circuit of an operating machine.
This solution enables to obtain short brake pedal strokes and the limited operating force that is typical of selector valves, but has the drawback of not providing any certainty as regards the strength of the braking power produced by the left and right circuits of the operating machine when the driver operates the brake pedals to brake on the road; therefore braking imbalances which cause swerving may also occur with this technical solution.
According to a further well-known technical solution, the brake pedals can jointly or individually operate a single pressure-modulating device positioned between the two brake circuits. It receives pressurised oil from an accumulator and sends it to both brake circuits of the operating machine by means of respective elements which open or close the passage of pressurised oil to the individual circuits.
The drawback of this known technical solution is that, if a leak accidentally occurs in just one of the two brake circuits, all of the oil pressure is discharged towards the exterior and the braking action is completely missed, causing serious danger.
According to another braking system for two parallel circuits two pressure-modulating devices are used, positioned in parallel and connected to one another via pressure ducts and/or valves; each modulating device can be operated individually using its respective pedal to operate the respective brake circuit and thus turn the operating machine.
When both pedals are operated together and both modulators are thus activated, the pressure that reaches the brakes is the highest pressure generated by the two modulators.
The oil in the two brake circuits is mixed between the two modulators and between the pressure ducts or valves connecting them.
Again, the drawback of this solution is that, as the modulators are connected to one another, in the event of an oil leak from one of the circuits the oil is lost from both, and the braking action is also missed.
Further to what has already been stated, in vehicles that are also equipped with brakes on the front wheels, the braking units on the front wheels are supplied, by means of a valve known as a reverse selector valve, by the two hydraulic circuits which control the respective brakes on the rear wheels.
The aim of this valve is to connect the brakes on the front wheels to the hydraulic circuit controlling the brakes of the rear wheels in which the oil pressure is lower; thus, if the driver operates both brake pedals, those operating the front wheels are also operated whereas, if just one pedal is operated and just one hydraulic circuit controlling the brakes on just one of the rear wheels is activated as a result, the brakes on the front wheels are connected to the inactive hydraulic circuit and thus also remain inactive.
The drawback of this solution is that if there is a breakage or oil leak in one of the hydraulic circuits controlling the rear wheels, the brakes of the front wheels cannot be operated either and the braking action only takes place on one rear wheel, thus unbalancing the vehicle.
Moreover, a further drawback is that if both pedals and therefore both brakes of the rear wheels are operated, but the pressure in one of the hydraulic circuits controlling the rear wheels is lower than in the other, there is a considerable delay in the operation of the brakes of the front wheels.